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Thomas Dam is one of the largest reservoirs in Kano, Nigeria and supports diverse fish species. Present study was carried out to assess the fish diversity status of the reservoir. Fish sampling was done monthly for a period of twelve months (January-December, 2022) using gill nets, cast nets, long lines, and different traps. A total of 1,318 fishes belonging to 6 families and 10 species were recorded during the study period. The results indicated Cichlidae as the dominant family represented by 4 species while Clariidae had representation with 2 species. Alestiidae, Mormyridae, Latidae, and Protopteridae were represented by 1 species each. Monthly fish diversity analyzed using nMDS (stress=0.06031) indicated influence of seasonality on the biodiversity of the Dam. The analysis of similarity (ANOSIM, P < 0.008, R = 0.47) show significant difference in fish assemblage structure between rainy season (June-September) and dry season (October-May). Also, Similarity percentage (SHRIMPER) analysis indicated 74.40% cumulative dissimilarity among the two Seasons contributed by five species; Protopterus annectens (29.85%), Brycinus nurse (15.47%), Oreochromis aureus (12.56%), Late niloticus (8.44%) and Sarotheroden galilaeus (8.08%). The fish diversity indices were calculated using PAST (version 3.22) software. Result of the diversity indices indicated Simpson’s diversity Index (1-D) and Shannon-Wiener index (H) values were highest during the months of rainy season of (1-D=0.85-0.89, H=2.06-2.24) indicating influence of the season on the fish diversity. The study concluded on the fact that changes of seasons have impact on the fish abundance and distribution of the Dam.